1. Field of the Invention
The present invention relates to an optical article provided with an optical coating and having excellent heat resistance.
2. Description of the Prior Art
In order to impart various desired properties to an optical article, it is common to employ an optical interference coating prepared by laminating a single layer or multi-layers of dielectric material having a high, intermediate or low refractive index.
For instance, there may be mentioned an antireflection coating as an example of commonly employed optical interference coatings. As an antireflection coating, there has been known a single-layered antireflection coating prepared by forming MgF.sub.2 (material having a low refractive index) on a substrate in an optical layer thickness of .lambda./4, or an antireflection coating having a three-layered structure prepared by forming on a substrate Al.sub.2 O.sub.3 (material having an intermediate refractive index), ZrO.sub.2 +TiO.sub.2 (material having a high refractive index) and MgF.sub.2 in an optical layer thickness of .lambda./4, .lambda./2 and .lambda./4, respectively, in this order from the substrate side. Further, as an optical filter, there has been known a filter prepared by laminating MgF.sub.2 and TiO.sub.2 (material having a high refractive index) alternately.
These optical interference coatings have sufficient mechanical strength, i.e. adhesion and hardness, for use at a temperature around room temperature, if they were formed by vapor deposition on substrates at e.g. 300.degree. C. However, there has been a problem such that if they are subjected, after the vapor deposition, to heat treatment e.g. at 450.degree. C. for 2 hours in air, and then they are used at a temperature around room temperature, the mechanical strength of the above-mentioned conventional coating structure tends to deteriorate.
For instance, as shown in FIG. 1, a glass plate with an antireflection coating is prepared by forming on a float glass substrate surface 1 an Al.sub.2 O.sub.3 layer 2 (thickness: 780 .ANG.) as the first layer, a ZrO.sub.2 +TiO.sub.2 layer 3 (ratio of ZrO.sub.2 /TiO.sub.2 : about 9, thickness: 1200 .ANG.) as the second layer and a MgF.sub.2 layer 4 (thickness: 940 .ANG.) as the third layer in this order by a vacuum vapor deposition method. Non-treated Sample 5 thereby obtained and Sample 6 obtained by the heat treatment thereof at 450.degree. C. for 2 hours in air, are subjected to abrasion resistance tests (eraser test and kaolin test) and scratch resistance test, as mechanical strength tests. The results are as shown in Table 1, and a distinct deterioration is observed in the heat-treated Sample 6 as compared with the non-treated Sample 5.
As a result of extensive researches on this deterioration mechanism, the present inventors have found the following facts. Namely, this deterioration is observed at the first interface from the air side, i.e. at the interface between the outermost MgF.sub.2 layer 4 and the ZrO.sub.2 +TiO.sub.2 layer 3 therebeneath. It is believed that such deterioration is caused by the difference in the thermal expansion coefficient between the MgF.sub.2 layer and the ZrO.sub.2 +TiO.sub.2 layer. In general, most of oxides have a thermal expansion coefficient not higher than 10.times.10.sup.-6 deg.sup.-1 (temperature range: room temperature to 450.degree. C.), while fluorides such as MgF.sub.2 have a thermal expansion coefficient of at least 20.times.10.sup.-6 deg.sup.-1 (temperature range: room temperature to 450.degree. C.) This difference in the thermal expansion coefficient is believed to cause slipping of the interface at the time of the heat treatment, whereby the bondage at the interface will be broken, and the bond strength at the interface will be weakened.
In the above-mentioned multi-layer type optical interference coatings, such disadvantages may be solved by employing, instead of the highly heat-expansive material MgF.sub.2, a material having a heat expansion coefficient smaller than that of MgF.sub.2. However, no other materials have so far been known which have adequate mechanical strength and durability by itself and chemical stability, and yet has a low refractive index comparable to MgF.sub.2. On the other hand, for the same reason, it is obliged to use an oxide material as a material having a high refractive index, whereby it is impossible to eliminate the difference in the heat expansion coefficient.